The present invention relates to porous carbon particles with a core/shell structure, comprising a core of a first carbon variety with a first porosity and a shell which surrounds the core and consists of a second carbon variety with a second porosity which is lower than the first porosity.
Furthermore, the present invention relates to a method for producing a porous carbon product having a core/shell structure. The method comprises the following steps:    a) providing a pore-containing template material consisting of inorganic material;    b) infiltrating the pores of the template material with a first precursor for carbon of a first variety;    c) carbonizing the first precursor so that, in the pores, a first deposit of the carbon of the first variety is formed with a first porosity;    d) infiltrating pores remaining after carbonizing with a second precursor for carbon of a second variety;    e) carbonizing the second precursor, wherein a second deposit of the carbon of the second variety is formed with a second porosity lower than the first porosity; and    f) removing the template material.
In the course of the development of portable electronic devices, the demand for rechargeable batteries (“accumulators” or secondary batteries”) is increasing. Fundamental requirements are high cell voltage, high charging capacity at an equivalence weight which is as low as possible. Moreover, a long cycle life is required, i.e. a small charging loss during charging and discharging.
Recently, lithium secondary batteries have gained technical importance. In these batteries, a cathode (positive electrode) and an anode (negative electrode) are provided consisting of a material which is suited for the insertion and removal (intercalation and de-intercalation) of lithium ions, and which adjoins an electrolyte which allows the movement of the lithium ions. As anode material, porous carbon structures are used that can reversibly incorporate and release lithium ions without the structural and electrical properties thereof being changed to a considerable extent.
Moreover, lithium-sulfur secondary batteries are being developed and are considered to be one of the very promising secondary batteries of the next generation. In its simplest configuration, the cell consists of a positive electrode of sulfur and of a negative electrode of lithium. The theoretical capacity is 1,675 mAh/g, on the assumption that all sulfur atoms are completely reduced to S2− upon discharge of an electrode, and the rated voltage is 2.2 V/cell. The component sulfur which is involved in the reaction (or sulfur-containing organic compounds) acts as an electrical insulator, so that the progress of an electrochemical reaction requires a permanent intimate contact with an electrically highly conductive component, such as carbon.
To ensure an electrical or ionic conduction of the electrodes, liquid electrolytes, often polar organic solvents, are used. These serve not only as ion transport media between the anode and cathode, but also as ion conductors within a sulfur-containing electrode.
This poses, on the one hand, the problem that the electrode structure is to provide a large surface occupied with electrochemical active material and to allow an unhindered access of the electrolyte liquid to the active material. These requirements can be met by a so-called hierarchically structured porosity of the electrode material, wherein large surfaces are provided by pores in the nanometer range that are accessible via a continuous macroporous transport system for the electrolyte.
On the other hand, active material, such as sulfide and polysulfide discharge products, can dissolve in the electrolyte and can be discharged thereby. The components diffused away from the electrode are no longer available for the further electrochemical reaction, whereby the charging capacity is decreasing. It is also possible that discharge products are irreversibly precipitated out of the electrolyte solution, whereby the charging capacity is also decreasing.
These disadvantageous effects are avoided with an electrode which contains porous carbon particles with core/shell structure according to the above-mentioned type, as is known from International Application Publication No. WO 2012/136513 A1. The known porous carbon particle comprises an inner layer of a first carbon variety which adjoins a cavity and which is in contact with an outer layer of a second carbon variety, the inner layer consisting of non-graphitic carbon and having a higher porosity than the outer layer.
For the preparation thereof, a pore-containing template material is used in the form of a SiO2 soot produced by gas phase deposition. In the pores of the SiO2 template material, two carbon varieties that differ from each other in their porosity are deposited one after the other, based on a two-stage infiltration process. In the first infiltration stage, the pores are infiltrated with a carbon precursor in the form of a carbohydrate solution, such as a sugar-water solution which, after carbonizing, yields non-graphitic carbon with a high porosity. The pore volume remaining or released after carbonizing, the first carbon precursor (C precursor) is homogeneously infiltrated in the second infiltration stage with a different carbon precursor, such as liquid pitch, which after carbonizing yields graphite-like carbon of a porosity lower than the porosity of the non-graphitic carbon. “Precursor for carbon” or “C precursor” stands here for a carbonaceous compound which can be deposited as a deposit on a substrate and converted by way of carbonizing into carbon. The carbonizing of the first C precursor leads to a carbon with a first porosity. The carbonizing of the second C precursor leads—under identical conditions of the carbonizing process—to a carbon with a second porosity lower than the first porosity.
After removal of the template material, a carbon structure of hierarchical porosity is obtained. The removed template material leaves cavities in the mesopore and macropore range that are three-dimensionally interlinked via the former sinter necks. Mesopores are typically of a pore size in the range of 2 nm to 50 nm. The carbon structure surrounding this linked cavity consists of a multi-layered carbon layer. The inner layer facing the cavity consists of predominantly non-graphitic carbon of a high microporosity. The low-porosity carbon of the outer layer reduces the microporosity and the specific surface area of the composite material on the whole without impeding the accessibility of a liquid electrolyte to the high-porosity turbostratic carbon of the inner layer. This structure of the carbon skeleton can be called “core/shell structure”. It is suited for retaining electrode material.
The publication of Marek Eder et al., “Thermal Decomposition of Petroleum and Coal Tar Pitches by Thermogravimetry”; in: Die Angewandte Makromolekulare Chemie, 1 Jan. 1986 (1986-01-01), pages 91-101, relates to a thermogravimetric analysis of coal tars and petroleum pitch in air and in nitrogen. The reaction order and the activation energy are determined for each decomposition stage. This yields data on the weight loss of the different pitches.
The Master-of-Science thesis by Segaula Isaac Manabile, “Study of the early stages of carbonisation of some pitch materials of different composition,” 31 Jul. 2009 (2009-07-31), pages 1-104, University of Pretoria (URL: http://upetd.up.ac.za/thesis/available/etd-11292009-204923/), relates to the origin and development of the mesophase in the case of different pitch modifications. Results of FT-IR studies and thermogravimetric measurements are described.
The publication of Yu Lei et al., “Porous mesocarbon microbeads with graphitic shells: constructing a high-rate, high-capacity cathode for hybrid supercapacitor,” in: SCIENTIFIC REPORTS, Vol. 3, 21 Aug. 2013 (2013-08-21), describes a porous carbon product with core/shell structure which is produced from mesocarbon microbeads (MCMB). The core consists of amorphous porous carbon and the shell consists of graphitic carbon.
The publication of Chang Song et al., “Hierarchical Porous Core-Shell Carbon Nanoparticles,” Chemistry of Materials, Vol. 21, No. 8, 28 Apr. 2009 (2009-04-28), pages 1524-1530, discloses carbon nanoparticles of a mesoporous core and a microporous shell. The mesoporous walls consist of a few graphene layers which leave hollow carbon nanoparticles after removal.
U.S. Pat. No. 6,355,377 B1 discloses a negative/active material for rechargeable lithium batteries which consist of a core of crystalline carbon and a shell of semi-crystalline carbon, the shell enclosing metal borides and metal carbides and comprising a turbostratic layer.
In the method known from International Application Publication No. WO 2012/136513 A1, the solvent-containing cane sugar layer which is deposited on the template material is dried and carbonized by heating in nitrogen at 700° C. By evaporation of the water and by carbonization, it loses up to 75% of its original mass. Therefore, the infiltration process leads in principle to a small thickness in the range of a few nanometers for the deposited turbostratic carbon layer. At least 50% of the remaining pore volume is occupied with carbon of the second variety.
It has been found that in the known carbon particles, the porosity of the turbostratic carbon of the inner layer which is distinguished by micropores with pore sizes in the range of less than 2 nm and also the mass distribution of the carbon varieties within the carbon structure are not optimal with respect to retention capacity for active material and charging capacity of the battery produced therefrom.
It is therefore an objective of the present invention to provide porous carbon particles with a core/shell structure and hierarchical porosity that exhibit a high retention capacity for active material and are distinguished by a high capacity and a low capacity loss during use as electrode material for a lithium or lithium-sulfur secondary battery.
Moreover, it is an objective of the present invention to indicate a method for the low-priced production of such carbon particles.